By proper choice of design frequency, one may have an SWR on one band edge equal to that of the opposite band edge. If this is your preference, you will want to establish a design frequency somewhere in the top half of the band. As an example, for the 75-80 meter band, the design frequency should be about 3.920 MHz. At the 4MHz end of the band the SWR will be about 1.5-1 with the SWR at 3.5 MHz at about 1.5-1. This is all without the aid of a 'match box'. However, antenna parameters may vary slightly from one antenna to another.
There are at least several reasons why this antenna is so broad band. One reason being this dipole is matched to the feed line. Another reason is it electrically incorporates its own balun. Also, this dipole has a large circular area over its entire surface, thus a low Q.
Over a 5 year period of testing, K7UAE, reports an arithmetic mean average of 1.5db gain over a simple dipole out to the same frequency and instlled at the same height and configuration. From the authors findings, this gain figure could be a conservative representation. Also, he reports a -6db noise figure due to static charge build-up common to open wire construction of the simple dipole. Since this coaxial dipole is completely covered by a vinyl jacket, it greatly reduces static charge build-up which discharges causing a popping noise in the receiver. The vinyl jacket covering is usually found on most well designed mobile antennas. This antenna also greatly reduces harmonics of the operating frequency. Any signal fed to the antenna which is harmonically related to the antenna's operating range is reduced by considerable amount as compared to a simple dipole. This feature, as well as other features of this antenna, amount to a substantial savings in cost for the extras such as a "match box", low pass filter, balun, and etc.
In summary, the advantages of this antenna are: 1} Broad bandwidth 2) Almost unaffected by environment 3} Positive gain with reference to a common dipole operating under the same relative conditions 4} Greatly attenuates harmonics 5} Substantial decrease in static charge build-up 6} Essentially non-directional.For antenna lengths, see diagram page.
For illustration, however, the 80 meter antenna will be used. It is suggested, at this time, one may use any 52 ohm coaxial cable for both the construction of the antenna and the feeding of it. The common choice of cables that meet this requirement are RG58A/U or RG8/U. It may be noted that RG58A/U is a very good choice in that it is the least expensive. As for attenuation in this coax, within the high frequency band, losses are considered negligible. Also, with this antenna, one may use the RG58A/U at the maximum legal power without fear of feed-line breakdown
For 80 meters, measure from this reference point out on each side of the center, 30 feet 6 inches and solder the inner conductor to the outer conductor of the coax. This forms the 52 ohm matching section and the balun. When completed one will want to waterproof these points of the antenna well. Next, at both ends of the antenna, the inner conductor must be shorted to the outer conductor. Waterproofing of the ends is not necessary at this time s the ends may need cutting later for tuning purposes.
At the center of the antenna, remove one inch maximum of vinyl jacket (« inch each side of center). Cut the shield in the center all the way around the coax, DO NO CUT the insulation or the center conductor. Form 2 leads with the shield. These leads re the feed point of the antenna. Next, connect the feed-line to these leads by soldering the feed-line center conductor to one lead and taping, then soldering the shield to the remaining lead. When this is completed, waterproof as best you know how. As for feed-line length, random length may be used. However, lengths of 57 feet, 87 feet, and 103 feet are about optimum for 75-80 meter use. The same coax type should be used for both antenna and feeding. Separate feed-lines must be used for each antenna.
Next, the antenna is erected. After erecting the antenna, measure SWR and trim the antenna to length at desired resonance. The SWR will be absolutely flat 1:1 when the antenna is at the proper length at design frequency. Be sure to solder the ends of the antenna (shield to center conductor) before measuring the SWR and also after completion of "tuning/trimming" process.
That is it! See the DIAGRAM page for diagrams of antennas at different frequencies. THIS ANTENNA may be used as a "flat-top" or in an inverted V fashion.
Best of 73's
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